Signing array of data using SHA1 with RSA in C

Question

I have to sign an array of data using SHA1 with RSA signature, with my private key in C. I am using OpenSSl libraries. However i always get no OPENSSL_Applink error. I have tried following:

• include applink.c
• static link libraires only libcrypto-1_1.a
• debug applink.c to see the problem with the table
• since i coundn't include applink.c i manually added the source code in the same filename in my project, which resulted in a non-responsive application.

I am using eclipseCDT oxygen version. I am a begginer in openSSl libraries and also would like if someone knows a good example of doing it to send me the link. I also tried adding OpenSSL_add_all_digests(), and OpenSSL_add_all_algorithms(), with no success.

Note: The code here may be wrong, but my problem is when compiler hits first custom function of openssl library project crashes with error no OPENSSL_Applink, so that's my main problem, not fixing the code itself.

Here is a code snippet:

int rsaSign(char *in_file, char * sig_file){

    char *data = NULL;
    int data_len;
    unsigned int sig_len;
    unsigned char *sig;
    int err = -1;

    OpenSSL_add_all_digests();
    FILE *fd;
    EVP_PKEY *priv_key = EVP_PKEY_new();
    RSA *privkey = NULL;
       printf( "we are here..\n");

    if ((fd = fopen(PRIVKEY_FILE, "r")) == NULL){
        printf("error reading file\n");
        exit(0);
    }

    privkey = RSA_new();
    if (!PEM_read_PrivateKey(fd, &privkey, NULL, NULL))
    {
        fprintf(stderr, "Error loading RSA Private Key File.\n");
        return 2;
    }

    fclose(fd);

    if (!EVP_PKEY_assign_RSA (priv_key, privkey))
    {
        fprintf(stderr, "EVP_PKEY_assign_RSA: failed.\n");
        return 3;
    }

    if (!priv_key) {
        printf("no private key\n");
    }
    EVP_PKEY_set1_RSA(privkey, priv_key);


    EVP_MD_CTX *ctx = EVP_MD_CTX_create();

    const EVP_MD *md = EVP_get_digestbyname("SHA1");

    if (!md) {
        fprintf(stderr, "Error creating message digest");
        fprintf(stderr, " object, unknown name?\n");
        ERR_print_errors_fp(stderr);
        exit(1);
    }



    if (!EVP_SignInit(ctx, md))
        {
            fprintf(stderr, "EVP_SignInit: failed.\n");
            EVP_PKEY_free(priv_key);
            return 3;
        }
    printf( "now to sign update..\n");
    data = readFile(in_file);
    data_len = strlen(data);
    printf("data len = %d\n", data_len);

    if (!EVP_SignUpdate(ctx, data, data_len))
    {
        fprintf(stderr, "EVP_SignUpdate: failed.\n");
        EVP_PKEY_free(priv_key);
        return 3;
    }
    printf( "now to sign final..\n");

    sig = malloc(EVP_PKEY_size(privkey)); //!!!!! SEGMENTATION FAULT HERE !!!!!


    if (!EVP_SignFinal(ctx, &sig, &sig_len, priv_key))
    {
        fprintf(stderr, "EVP_SignFinal: failed.\n");
        free(sig);
        EVP_PKEY_free(priv_key);
        return 3;
    }

    free(data);
    free(sig);
    EVP_PKEY_free(priv_key);
    return EXIT_SUCCESS;

    }

Answer 1

Your code passes &privkey which is type RSA** to PEM_read_PrivateKey which requires (per its prototype) EVP_PKEY**; your compiler should have diagnosed this. Similarly EVP_PKEY_size requires EVP_PKEY* not RSA*. With the generic read, you don't need any RSA* at all and need not and must not use EVP_PKEY_assign_RSA.

Applink should work if your code (including all calls to OpenSSL AND the #include "applink.c") is in the EXE not in a DLL -- is it?

However, you can eliminate the need for Applink even with dynamic linking by avoiding APIs that use FILE* and using the BIO* forms. Thus instead of:

FILE* f = fopen (file,"r");
if( f==NULL ) ...error... 
... PEM_read_PrivateKey (f, &evpkey, NULL, NULL) ...
fclose(f);

use:

BIO* b = BIO_new_file (file,"r");
if( b==NULL ) ...error...
... PEM_read_bio_PrivateKey (b, &evpkey, NULL, NULL) ...
BIO_close(b);

or possible but clunkier:

FILE* f = fopen (file,"r"); if( f==NULL ) ...error...;
char* buf = ...read all data from f into allocated memory...;
int len = ...length of data in buf...;
fclose (f);
BIO* b = BIO_new_mem_buf (buf,len);
... PEM_read_bio_PrivateKey (b, &evpkey, NULL, NULL) ...
... free buf as appropriate ...

PS: I don't know if the MinGW package has man pages, but most Windows packages (other than Cygwin and now WSL) don't. If not you should probably bookmark and use the appropriate directory(ies) under https://www.openssl.org/docs/manpages.html .